The present invention relates generally to active-matrix liquid crystal display devices and, more particularly, to liquid crystal display devices of the lateral electric field type having wide view-angle characteristics suitable for improvement of the aperture ratio.
Liquid crystal display devices of the active matrix type, which employ active elements typically, including thin-film transistors (TFTS), are becoming more important in the manufacture of display terminals for use with OA equipment in view of the fact that these devices offer enhanced displayability with superior image quality in comparison to cathode ray tubes, not to mention the flatness and light-weight features thereof. Such liquid crystal display devices are generally categorized into two types.
In one type, a liquid crystal material is sandwiched between two substrates, with a plurality of transparent electrodes being arranged thereon, so that application of a voltage to such transparent electrode causes an electric field to be generated transverse to the substrate, thereby modulating rays of light falling onto the liquid crystal after passing through the transparent electrodes, to thereby generate a displayxe2x80x94all of the currently available products are designed to employ this scheme.
The other type of device was a scheme for causing the liquid crystal to be modulated by an electric field that is generated substantially in parallel to a substrate surface between two electrodes arranged on the same substrate, thereby modulating light incident on the liquid crystal from a space between the two electrodes, to thereby generate a display in which the viewing angle is extremely wide. This technology, which shows great promise for improvements in active-matrix liquid crystal display devices, is called a xe2x80x9clateral electric fieldxe2x80x9d type or, alternatively, an xe2x80x9cin-plane switchingxe2x80x9d type device.
Some features of the latter type of device have been disclosed in Domestically Published Japanese PCT Application No. 5-505247 Published Japanese Patent Application No. 63-21907 (JP-A-63-21907), and JP-A-6-160878.
However, in the in-plane switching type device, since an opaque metal electrode is arranged into a comb-like shape on one substrate, the resulting ratio of the opening region permitting light to pass therethrough (aperture ratio) is significantly low, which results in the problem that active-matrix liquid crystal display devices of the in-plane switching type have a display screen which is dark, or, alternatively, a bright backlight with great power dissipation must be used in order to brighten the display screen, resulting in an increase in the power dissipation of the devices.
Another problem associated with the in-plane switching type device is that the use of a metal electrode leads to an increase the reflectivity at the electrode, which in turn creates a problem in that an image or the like appears like a ghost image on the screen due to reflection at the electrode, reducing the recognizability of the display.
The present invention is designed to solve the problems mentioned above, and an objective of the present invention is to provide an active-matrix liquid crystal display device employing the in-plane switching scheme, which device is capable of realizing a viewing angle equivalent to that of cathode ray tubes, and wherein the active-matrix liquid crystal display device is bright due to a high aperture ratio and yet is low in power dissipation and in reflection for achieving increased displayability.
To attain the foregoing object, the present invention offers, as its first aspect, an arrangement in which at least one of a pixel electrode and a counter electrode is provided as a transparent electrode; the normally-black mode is established for providing dark display in the absence of an electric field as applied thereto; the initial alignment state of the twistable liquid crystal layer is the homogeneous alignment state upon application of no electric fields; liquid crystal molecules between said electrodes and those on the electrodes upon application of an electric field rotate controllably in a direction substantially parallel to the substrate surface; the maximum value of the optical transmissivity of a liquid crystal display panel is 4.0% or greater; and, the view-angle range of the contrast ratio of 10 to 1 or greater is within the range of all-directional coverage as tilted by 40 degrees or more from the vertical direction relative to the display plane.
As a second aspect of the invention, at least one of the pixel electrode and counter electrode is provided as a transparent electrode, the normally-black mode is set for providing dark display upon application of no electric fields, the initial alignment state of the twistable liquid crystal layer is the homogeneous state upon application of no electric fields, and the twist elastic modulus is not greater than 10xc3x9710xe2x88x9212 N (Newton).
As a third aspect of the invention, at least one of the pixel electrode and counter electrode is provided as a transparent electrode, the normally-black mode is set for providing dark display upon application of no electric fields, the initial alignment state of the twistable liquid crystal layer is the homogeneous state upon application of no electric fields, the initial pretilt angle of those liquid crystal molecules at the upper and lower interfaces of the liquid crystal layer is not more than 10 degrees, and the initial tilt state of liquid crystal molecules within the liquid crystal layer are in the splay state.
As a fourth aspect of the invention, at least one of the pixel electrode and counter electrode is provided as a transparent electrode, the normally black mode is set for providing dark display upon application of no electric fields, the initial alignment state of the twistable liquid crystal layer is the homogeneous state upon application of no electric fields, and the average tilt angle of liquid crystal molecules of the liquid crystal layer on the transparent electrode is less than 45 degrees even when applying an electric field thereto.
As a fifth aspect of the invention, in any one of the arrangements described above, a double structure of a transparent electrode and an opaque electrode is employed for at least either the pixel electrode or the counter electrode.
As a sixth aspect of the invention, in any one of the arrangements described above, a structure is used in which neighboring ones of contra-voltage signal lines are connected by a counter electrode within a pixel via more than one through-hole.
As a seventh aspect of the invention, in any one of the arrangements described above, a protective film is provided for use in covering or coating active matrix elements, and at least one of said pixel electrode or said counter electrode is formed overlying said protective film while permitting electrical connection via more than one through-hole as formed in said protective film to either active matrix elements or contra-voltage signal lines.
As an eighth aspect of the present invention, in any one of the arrangements described above, the counter electrode is made of a transparent electrode, and further use is made of a structure having an optical shield pattern between a counter electrode and an image signal line.
As a ninth aspect of the invention, in any one of the arrangements described above, the contra-voltage signal line for electrical connection between counter electrodes is made of a metal.
As a tenth aspect of the invention, in any one of the arrangements described above, more than three counter electrodes are formed, two of which are formed adjacent to image signal lines, wherein the counter electrodes formed adjacent to the image signal lines are opaque.
As an eleventh aspect of the invention, in any one of the arrangements described above, a transparent conductive film for use as the transparent electrode is made of indium-tin-oxide (ITO).
As a twelfth aspect of the invention, the contra-voltage signal line is made of Cr, Ta, Ti, Mo, W, Al, or an alloy thereof, or, alternatively, a clad structure with such materials laminated.
As a thirteenth aspect of the invention, the contra-voltage signal line is a clad structure with a transparent conductive film such as indium-tin-oxide (ITO) or the like being laminated on Cr, Ta, Ti, Mo, W, Al, or an alloy thereof.
As a fourteenth aspect of the invention, in any one of the arrangements described above, the initial twist angle of said liquid crystal layer is substantially zero, wherein the initial alignment angle is greater than or equal to 45 degrees and yet less than 90 degrees when the dielectric anisotropy xcex94∈ of the liquid crystal material is positive in polarity, whereas it goes beyond zero degree and stays less than 45 degrees if the dielectric anisotropy xcex94∈ is negative.
As a first manufacturing method, the invention is featured by forming at least any one of a scan signal line end section, an image signal line end section or the uppermost layer of a counter electrode end section and at least one of a pixel electrode or counter electrode as a transparent conductive layer, and further by forming them in the same process step.
An example of the features of the present invention will be set forth below.
First of all, according to the first aspect of the invention, at least one of the pixel electrode or its counter electrode is made transparent to increase the light penetrating such portion, thereby to effect improvement of the maximum optical permeability or transmissivity during bright (white) display and make it possible to produce a brighter display than in cases where the electrodes are opaque so that the liquid crystal display panel""s transmissivity can be improved in the value of the maximum transmissivity from 3.0 to 3.8%, in the case of employing opaque electrodes, to up to 4.0% or greater in accordance with the present invention. More specifically, assuming that the brightness or luminance of backlight incident light is at 3,000 cd/m2 the maximum brightness value of bright-display luminance can attain 120 cd/m2 or greater.
Further, as the liquid crystal molecules retain their initial homogeneous alignment state upon application of no voltages, when the layout of polarizer plates is designed to establish dark (black) display in such state (in normally-black mode), no rays of light pass through such portion even where the electrodes are made transparent, thereby making it possible to achieve a dark display of good quality, thus improving the contrast.
On the contrary, if the normally-white mode is set then dark displaying must be carried out upon application of a voltage, which results in an inability to completely block the light at portions overlying the electrodes upon application of a voltage, which in turn makes it impossible to provide a dark display with good quality due to the fact that the transmitted light at such portions increases the transmissivity of the dark display. For this reason, a sufficient contrast ratio cannot be attained.
Furthermore, wide viewing angle characteristics can be obtained because those liquid crystal display molecules between said electrodes and over the electrodes upon application of a voltage thereto behave to controllably rotate in a direction parallel to the substrate surfaces.
Accordingly, wide view-angle characteristics can be obtained in which the view-angle range of contrast ratios of 10 to 1 or more falls within an all-directional range with an inclination of 40 degrees or greater from the vertical direction with respect to the display plane.
According to the second aspect of the invention, the twist elastic modulus of a twistable liquid crystal layer is less than or equal to 10xc3x9710xe2x88x9212 N (Newton) when applying a voltage between the pixel electrode and counter electrode, the angle xcex1 of rotation from the initial alignment direction increases on or over a transparent conductive film to allow the on-electrode transmissivity to complementally interact with the transmissivity between electrodes to substantially improve the aperture ratio. It is preferable that this twist elastic modulus K2 be smaller.
According to the third aspect of the invention, in view of the fact that the initial pretilt angle of liquid crystal molecules at the upper and lower interfaces of a liquid crystal layer is less than or equal to 10 degrees, while the initial tilt state of liquid crystal molecules inside of the liquid crystal layer is in a splay state, the tilt angle of liquid crystal molecules at the center of the liquid crystal layer becomes nearly zero degrees to thereby enable the liquid crystal layer contributing to the display to decrease in average tilt angle; thus, even upon application of a voltage, it becomes possible to establish low tilt angles of those liquid crystal molecules between electrodes and over transparent electrodes, which in turn makes it possible to realize both aperture ratio improvement and wide viewing angles.
According to the fourth aspect of the invention, both aperture ratio improvement and wide viewing angles can be realized due to the fact that the average tilt angle of the liquid crystal layer""s liquid crystal molecules on or over the transparent electrode stays below 45 degrees even when applying a voltage thereto.
According to the fifth aspect of the invention, the use of a double or duplex structure of a transparent electrode and opaque metal electrode for either the pixel electrode or the counter electrode makes it possible to greatly prevent short-circuiting defects at this electrode, which will be advantageous for achievement of large screens.
According to the sixth aspect of the invention, the use of a structure for letting neighboring contra-voltage signal lines be connected by a counter electrode within a pixel via more than one through-hole permits respective contra-voltage signal lines to be electrically connected together in a net-mesh-like pattern, which makes it possible to reduce the resistivity of such contra-voltage signal lines, wherein serious defects will no longer take place even upon occurrence of open circuit failures.
The seventh aspect of the invention lies in an ability to let the protective film suppress reduction of an electric field acting on liquid crystal molecules, which makes it possible to lower the drive voltage(s).
According to the eighth aspect of the invention, the aperture ratio is improved by use of a structure in which the counter electrode made of a transparent electrode and an optical shield pattern is provided between the counter electrode and its associative image signal line(s).
According to the ninth aspect of the invention, lowering the resistivity of contra-voltage signal lines makes it possible to smoothen the transmission of a voltage between counter electrodes, thus reducing distortion of the voltage, which in turn enables suppression of cross-talk in the horizontal direction.
According to the tenth aspect of the invention, by making the counter electrode neighboring upon image signal lines opaque will suppress crosstalk associated with image signals. The reason for this is set forth below.
Forming a transparent counter electrode in close proximity to an image signal line forces an electric field (electric flux lines) from the image signal line to be absorbed by the counter electrode, with a result that the electric field from the image signal line hardly affects the electric field generated between the pixel electrode and counter electrode to thereby extremely suppress generation of crosstalk associated with image signals-in particular, crosstalk in the up/down direction of the substrates concerned. However, the behavior of the liquid crystal molecules on or over the counter electrode neighboring upon the image signal line is unstable due to variation of image signals; and, if the counter electrode that neighbors the image signal line is made transparent, then crosstalk is observed due to transmitted light at such an electrode portion. Accordingly, letting the counter electrode adjacent to the image signal line be opaque makes it possible to suppress crosstalk associated with image signals.
According to the eleventh aspect of the present invention, the transparent conductive film is indium-tin-oxide (ITO), which is suitable for improvement of the optical transmissivity.
According to the twelfth and thirteenth aspects of the present invention, the contra-voltage signal line is a laminated clad structure, and the resistance value decreases enabling reduction of open circuit defects.
According to the fourteenth aspect of the present invention, because the liquid crystal layer""s initial twist angle is nearly zero, while the initial alignment angle is greater than or equal to 45xc2x0 C., and yet is less than 90xc2x0 C. if the dielectric anisotropy xcex94∈ is positive in polarity and is above 0xc2x0, and yet is less than or equal to 45xc2x0 if the dielectric anisotropy xcex94∈ is negative, it is possible to improve the contrast by suppressing the domain and optimizing the range of a maximal application voltage, while at the same time enabling optimization of the response speed.
The first manufacturing method is designed to enable fabrication of pixel electrodes and counter electrodes using transparent conductive films without increasing the required number of process steps, by simultaneously forming both the transparent conductive layer of a scan signal line terminate end portion, an image signal line end, or the counter electrode end""s uppermost-layer and the transparent conductive film of the pixel electrode or counter electrode.
It should be noted that although the liquid crystal display device of the present invention is designed so that at least one of the pixel electrode and the counter electrode is formed of a transparent conductive film, a difference in configuration from a liquid crystal display device as recited in, for example, Richard A. Soref, Proceedings of the IEEE, December issue, 1974 at pp. 1710-1711 (referred to as xe2x80x9cReference 1xe2x80x9d hereinafter) is as follows.
In Reference 1, a comb-shaped electrode corresponding to a pixel electrode and counter electrode is constituted from a transparent conductive film.
However, when forming the initial alignment state of liquid crystal molecules, SiO (silicon mono-oxide) is orthorhombically deposited at about 85 degree to intentionally form extremely high pretilt angles at the liquid crystal molecules in the interface between each electrode and the liquid crystal layer. For this reason, as shown in FIG. 1(b) of Reference 1, applying a voltage between comb-shaped electrodes from the homogeneous alignment with 90-degree twisting in the initial alignment state results in formation, as the realignment state, of a homogeneous alignment state that is substantially parallel to substrate surfaces in a region between the electrodes and of a homeotropic alignment state that is perpendicular to substrate surfaces in a region on or above the electrodes.
However, with this arrangement, there is a drawback in that, although complementary interaction of the two kinds of liquid-crystal molecule realignment states with an increase in electric field might result in achievability of brighter display, the resultant viewing angle characteristic becomes narrower due to a need to averagely increase the tilt angle of liquid crystal molecules.
On the contrary, with the liquid crystal display device of the lateral electric field type in accordance with the present invention, a specific configuration is employed wherein even when applying a voltage between the pixel electrode and counter electrode in order to obtain a wide view-angle characteristic and a good aperture ratio, those realigning portions of liquid crystal molecules contributing to a display image are forced to retain the homogeneous alignment state that maximally parallels the substrate surfaces while simultaneously letting, on or over electrodes of a transparent conductive film, the on-electrode transmissivity complementary interact with the interelectrode transmissivity in a way corresponding to the angle xcex1 of rotation from the initial alignment direction, resulting in substantial improvement of the aperture ratio.
It should be noted that in the description, the term xe2x80x9chomogeneous alignment statexe2x80x9d refers to a state in which the liquid crystal molecules within a liquid crystal layer have a tilt (rise-up) angle lying maximally parallel to either the substrate surface or the interface of such liquid crystal layer are practically, a specific alignment state in which the tilt angle from either the substrate surface or the liquid crystal layer""s interface stays below 45 degrees. Accordingly, the xe2x80x9chomeotropic alignment statexe2x80x9d is defined as a case in which the tilt angle from either the substrate surface or the liquid crystal layer""s interface exceeds 45 degrees.
FIG. 41A shows an example of a voltage potential distribution within a liquid crystal layer in an electrode arrangement for creation of an electric field extending nearly parallel to the substrate surface.
Solid lines in the drawing designate equal-potential lines, wherein an electric-field vector is given in a direction perpendicular to such equal-potential lines. While the electric field vector E permits production of only components Ey extending at right angles to the substrate surface on the electrode center, those components Ex extending horizontally relative to the substrate surface also appear in the remaining part other than the center. In a region in which such horizontal componentsxe2x80x94i.e. lateral electric field componentsxe2x80x94Ex are being generated, liquid crystal molecules between the electrodes behave to rotate through a rotation angle xcex1 from the initial alignment direction RDR in the direction of the lateral electric field Ex as shown in FIGS. 41B and 41C.
On the other hand, on-electrode liquid crystal molecules behave to rotate with a rotation of the interelectrode liquid crystal molecules in the presence of a molecular field. Accordingly, although no lateral electric field is being applied to the central on-electrode liquid crystal molecules, these molecules attempt to rotate due to the molecular field in the same direction as that of their outlying liquid crystal molecules. In other words, the rotation angle xcex1 is large between the electrodes, decreases at locations on or above the electrodes, and becomes maximal over the electrode center portion.
A result of simulating this manner of operation is shown in FIGS. 42A-42C.
Note here that the simulation in this example was carried out using an exemplary arrangement in which the liquid crystal molecules"" initial homogeneous alignment state is designed so that the liquid crystal layer""s initial twist angle is substantially zero, whereas an initial alignment angle defined between the initial alignment direction RDR and the applied electric field Ex is set at xcfx86LC=75 degrees, while letting the initial pretilt angle of certain liquid crystal molecules near or around the liquid crystal layer""s upper and lower interfaces be set at zero degrees, and further employing a Cross Nicol layout that lets the transmission axis of one of polarizer plates be identical to said initial alignment direction RDR with the transmission axis of the other polarizer plate being at right angles, thereby performing displaying in a double refraction mode.
The optical transmissivity T/T0 at this time may be represented by the following equation:
T/T0=sin2 (2xcex1eff)xc2x7sin2(Πdeffxc2x7xcex94n/xcex)xe2x80x83xe2x80x83(1)
Here, xcex1eff is the angle defined between the liquid crystal layer""s effective light axis and the polarized-light transmission axisxe2x80x94in this example, this is the net value of the liquid crystal molecule rotation angle xcex1 in the direction along the thickness direction of the liquid crystal layer, which is a xe2x80x9cvirtualxe2x80x9d value that is treatable as the average value under an assumption that the rotation is uniform.
Additionally, deff is the effective thickness of a liquid crystal layer having double-refractivity, xcex94n is the refractive anisotropy, and xcex is the wavelength of light.
In Equation (1), at the time of application of an electric field Ex, the value of xcex1eff increases with an increase in the intensity thereof, and becomes maximal at 45 degrees.
Furthermore, in the simulation of this example, the liquid crystal layer""s retardation xcex94nxc2x7deff is set at a selected value that is half of the wavelength xcex of light for achievement of the double refraction zero-order mode, while setting the dielectric anisotropy xcex94∈ to a positive polarity.
FIG. 42A is a characteristic diagram showing the state of equal-potential lines in the case of applying to a transparent ITO electrode a voltage at which bright display near the maximum is obtainable, wherein the vertical axis represents the thickness (4.0 xcexcm thick) of a liquid crystal layer and the transverse axis indicates a relative electrode positional relationship. Note that the values in this drawing are indicative of the voltage potential strength standardized.
Also see FIG. 42B and FIG. 42C, which show the rotation angle xcex1 and tilt (rise-up) angle of liquid crystal molecules within a liquid crystal layer upon application of lateral electric field components Ex as formed from the state of the equal-potential lines.
As shown in FIG. 42C, the on-electrode liquid crystal molecules hardly rise up even when applying a voltage thereto In this example, the tilt angle stays below 8xc2x0 in the entire direction along the thickness of the liquid crystal layer. Further, as shown in FIG. 42B, those liquid crystal molecules on or over the electrodes also have rotated about 15 to 35xc2x0. It is noted that the sign of the tilt angle shown in FIG. 42C is determined so that the rightward rise-up in the drawing is positive whereas leftward rise-up is negative for purposes of convenience in illustration and discussion herein. Therefore, with the scheme of the present invention, it becomes possible to allow the liquid crystal molecules to vary in rotation angle xcex1 even on or over the electrodes to thereby change the transmissivity.
The one character that is most pertinent to this operation is the liquid crystal is twist elastic modulus K2, which is preferably as small as possible in view of the fact that as this modulus K2 gets smaller, liquid crystal molecules on or over electrodes receive influence of the interelectrode liquid crystal molecules to rotate approaching the rotation angle xcex1 of such interelectrode liquid crystal molecules.
Referring to FIG. 41D, there is shown a model of a distribution of the on-electrode transmissivity and the interelectrode transmissivity in a case where the twist elastic modulus K2 is set at about 10xc3x9710xe2x88x9212 N (Newton).
In case the electrodes concerned are transparent, the on-electrode liquid crystal molecules"" realignment operation discussed above allows 5 to 30% of the average transmissivity of transmissivities at part xe2x80x9cAxe2x80x9d between electrodes to become the average-value transmissivity of transmissivities at part xe2x80x9cBxe2x80x9d on or over the electrodes.
In addition, as will be described later, it has been found that if the twist elastic modulus K2 is less than or equal to 2.0xc3x9710xe2x88x9212 N (Newton), then more than 50% of the average transmissivity of transmissivities at the part xe2x80x9cAxe2x80x9d between the electrodes becomes equal to the average-value transmissivity of transmissivities at the part B on or over the electrodes. Therefore, the average transmissivity over the entire part is raised up to become the average-value transmissivity of the transmissivities at the A+B portions.
In summary, when compared to electrodes which have been traditionally comprised of a metal layer that permits no light rays to pass through, it becomes possible to substantially improve the aperture ratio per pixel.
With the simulation of this example, calculation is carried out with the initial pretilt angle being set at zero degrees; however, in actual implementation, it will be required that the initial pretilt angle near or around the interfaces of the liquid crystal layer with its associative alignment film(s) be set by rubbing treatment at approximately 10 degrees or less; and, more preferably, it is set at 6 degrees or below. Additionally, in an embodiment to be later described, it is set at about 5 degrees.
With the initial pretilt angle falling within such a range, it is possible to control the liquid crystal molecules at the liquid crystal layer interfaces so that they align in the substrate in-plane direction, thereby making it possible to allow the average tilt angle of the liquid crystal layer on or over electrodes to stay below 45 degrees even upon application of electric fields thereto. In other words, it becomes possible even when applying electric fields to prevent on-electrode liquid crystals from exhibiting so-called homeotropic alignment.
FIG. 44 is an example of a characteristic diagram of a simulation result, which shows tilt angles of liquid crystal molecules within a liquid crystal layer in the liquid crystal display device of the lateral electric field type, along with a view-angle range in which the contrast ratio becomes 10 or greater in all directions concerned.
More specifically, even when the tilt angle is about 30 degrees, the resultant contrast ratio stays at or above 10 in all the directions within the view-angle range with about 40-degree inclination from the vertical direction relative to the display plane, which results in achievement of the intended characteristics that are substantially identical to those in prior art liquid crystal display devices of the longitudinal electric-field type. Furthermore, the less the tilt angle, the greater will be the view-angle range. If the former is about 10 degrees, then the latter expands to exhibit a view-angle range with inclination of about 80 degrees; whereas, if the former is 5 degrees or less, then the latter expands to fill almost the entire range-thus, wide view-angle characteristics are obtained.
In this embodiment, since this aspect of the invention is designed to reduce at any event the average tilt angle of the liquid crystal molecules within the liquid crystal layer between the electrode and on or over the transparent electrode when applying no electric fields and when applying an electric field thereto, the rubbing direction of alignment films ORI1, ORI2 to be later described are set in an initial alignment state so that the initial pretilt angle of the liquid crystal molecules at the interfaces of the liquid crystal layer on the sides of the two substrates SUB1, SUB2 is in a is state to thereby ensure that certain liquid crystal molecules at or near the center of the liquid crystal layer exhibit maximized parallelism with respect to the interfaces.